One hundred healthy Caucasian medical students (age 22 +/- 1 years) were vaccinated with a recombinant hepatitis B vaccine and their haptoglobin types were determined. A relationship between haptoglobin type and immune response to the vaccine was observed. Subjects with a 2-2 haptoglobin phenotype produced significantly lower hepatitis B antibody titres than those having a 1-1 or 2-1 haptoglobin phenotype. The haptoglobin phenotypes not only influenced the magnitude but also the kinetics of the anti-HBs response. For all haptoglobin types, haptoglobin concentration and immune response to the vaccine behaved independently.